For example, the refractive index of a lens determines how it could be used to magnify an image through a microscope or help someone see with glasses. And the refractive index is not just for glass but for gases, liquids, and solids of all sorts. A material can be modified to fine-tune its refractive index for a particular purpose. Making use of this characteristic, though, depends on being able to accurately and repeatedly measure it.
To measure a material’s index of refraction, scientists often use a temperature-controlled refractometer. “As the refractive index depends on the temperature, most of the applications require temperature control to get correct measurements,” says Christian Iserland, who manages various technologies, including refractometry, at METTLER TOLEDO (Greifensee, Switzerland). The level of temperature control also affects the results. For example, without accurate temperature control, the measurements can wander over minutes.
The speed of temperature control also matters. “Stabilizing the sample’s temperature quickly offers greater throughput of samples being read and also results in greater accuracy,” says Larry Pastwik, who handles the technical applications of analytical instruments at Reichert Technologies (Depew, NY). “The way in which accuracy is improved is by the sample being stable when capturing the refractive index value and providing a stable temperature value at the probe.”
When using Reichert’s AR7 series of temperature-controlled refractometers, Pastwik says, “The customer is assured of sample input values to the instrument’s software that have been captured in a timely manner and result in an accurate value being displayed.” That’s what a customer should look for in any device being considered.
To some extent, the pros and cons of refractometry come from the same thing. The joy of refractometry is that every molecule out there has a refractive index, so you can get a signal from anything, and the heartache is that you get a signal from everything. Therefore, you need the right method and the right device to ensure you get the signal that you want.
Although temperature affects all measurements of a sample’s refractive index, that doesn’t mean that everyone needs a temperature-controlled device. As an example, Kevin Gable, professor of chemistry at Oregon State University in Corvallis, says, “In our application—an undergraduate lab course—temperature control is not a major issue.” He adds, “We operate near room temperature, and the admittedly crude corrections for temperature are sufficiently accurate that we can meet characterization needs without careful temperature control.” Nonetheless, Gable’s students use additional methods, including gas chromatography and infrared detection to augment their measurements. As he says, “I think the issue would be more critical were we either characterizing new compounds or using refractometry as a sole means of establishing purity.”
Today’s temperature-controlled digital refractometers provide more opportunities through added technology. For instance, a sample changer can be added to METTLER TOLEDO’s LiquiPhysics Excellence refractometers. Then, Iserland says, scientists can “run a series of samples.” He adds, “Our OneClick can start a complete workflow including product quality control and export to LIMS/SAP.”
Whatever kind of instrument you’re using, the right care improves the accuracy of measurements. The fluid in the captured side must be flushed out once a week or so. Also, refractometers are incredibly sensitive to back pressure, so they need to be last in a line of analytical devices with relatively wide-bore tubing.
If a scientist keeps the device functioning properly and compensates for temperature in some way, refractometers can provide accurate measurements for many applications, despite this technology’s heartache.
For additional resources on refractometers, including useful articles and a list of manufacturers, visit www.labmanager.com/refractometers